1. Field of the Invention
The present invention relates to a process for forming a sheet of refractory fibers using foam. The present invention also relates to a novel agitator for use in foam-forming a wet-laid, sheet of refractory fibers. More specifically, the present invention relates to an efficient foam process for making a uniform sheet of refractory fibers.
2. Description of the Related Art
The use of foam in a furnish for preparing wet-laid, non-woven fibrous webs is known. See, for example, U.S. Pat. No. 4,443,297, which discloses an apparatus and method for the manufacture of a non-woven fibrous web using foam. The method involves preparing a foam furnish with 55-75% volume air, recirculating a surfactant-water solution through a forming wire followed by foam storage in a silo to eliminate excess air, and then recycling foam from the bottom of the silo. See also, U.S. Pat. Nos. 4,349,414; 4,443,299; 4,498,956; and 4,543,156.
U.S. Pat. No. 4,488,932 relates to a method of manufacturing fibrous webs of enhanced bulk. The method involves hammermilling dry hydrophilic fibers to generate crimp, and then foam forming these fibers in 0.5 to 5 minutes to retain as much crimp as possible.
U.S. Pat. No. 4,686,006 relates to an apparatus and method for laying down a fibrous web from a foam-fiber furnish. A headbox is used which includes walls defining an elongate channel extending transversely of the direction of movement of the forming wire. Foam forming nozzles are positioned to introduce foam-fiber furnish into the channel for turbulence, inducing impact on an oppositely disposed wall defining the channel. The turbulently flowing foam-fiber furnish is then introduced to the headbox slice for discharge onto the forming wire with minimized orientation of the fibers.
Other patents which relate to the use of foam in making non-woven fibrous webs include U.S. Pat. Nos. 3,716,449; 3,938,782; 3,871,952; 3,837,999; 3,876,498; 3,846,232; 4,062,721; 3,746,613, 4,056,456; 5,720,851; 5,904,809; 6,238,518 and 6,258,203.
Sheets of refractory fibers have been made. However, the formed sheets lack uniformly and the processes are inefficient. Improved uniformity of dispersion and distribution of the refractory fibers in the web would be a great step forward in the art, as would increased ease and efficiency in forming the web. Techniques useful in the formation of more uniform non-woven webs made of refractory fibers would be of great benefit to the industry as such refractory fiber sheets have many potential uses.
Accordingly, it is an object of the present invention to provide a novel process for forming a non-woven fibrous web of refractory fibers using foam, which process provides a web in which the fibers are uniformly and evenly distributed.
Yet another object of the present invention is to provide a novel agitator, which agitator is useful in preparing the foam furnish for forming a non-woven fibrous web of such refractory fibers.
These and other objects and features of the invention will become apparent to one skilled in the art upon a review of the following description, the figures of the drawing and the claims.
In accordance with the foregoing objects, there is provided by the present invention an effective and efficient method for preparing a non-woven fibrous web of refractory fibers using a foam furnish, which foam furnish is prepared by using the novel agitating means of the present invention.
The apparatus used for agitating the refractory fibers in a foamed medium is unique. The apparatus comprises agitating means mounted for displacement within a foamed medium and includes a leading surface facing in a direction of displacement, the leading surface including upper and lower portions converging in the direction of displacement to form a generally convex leading surface. The trailing surface is concave. The abrupt transition between the two surface shapes leads to cavitation or bubble formation. The apparatus further comprises driving means for displacing the agitating means in the direction of displacement for dispersing and mutually separating the refractory fibers within the foamed medium.
More specifically, the apparatus for agitating the fibers in a foamed medium comprises a tank having a cylindrical surface forming an agitating chamber for containing a mixture of refractory fibers and foamed medium, which can include other functional additives. The agitating means is mounted for rotation about an upright axis coinciding with a longitudinal axis of the agitating chamber and including a plurality of legs projecting generally radially from the axis, each leg including a leading surface facing in a direction of rotation and terminating in upper and lower trailing ends, said leading surface including upper and lower portions which converge in the direction of rotation to form a generally convex leading surface. The apparatus frrther comprises driving means for rotating the agitating means to disperse and mutually separate the refractory fibers within the foamed medium, the agitating means forming a central agitation zone, the ratio of the diameter of the agitation zone to the diameter of the agitating chamber being from about 0.5 to about 0.95.
Thus, by the present invention there is provided a method for forming a non-woven, fibrous web composed of refractory fibers which comprises first forming a foam furnish by agitating the refractory fibers in a foamed medium, preferably aqueous, with the agitating means of the present invention. The resulting foam furnish is then passed onto a screen and defoamed using conventional techniques.
In another embodiment of the present invention there is provided a non-woven fibrous web comprised of refractory fibers prepared by the method of the present invention. The web exhibits excellent uniformity, i.e., very few fiber bundles, and substantially no fiber directionality. The web can also be comprised of very long fibers, as the method of the present invention can easily, and with efficiency, handle even long refractory fibers.